Ion mobility spectroscopy (IMS), sometimes known as plasma chromatography, is a technology that is ideally suited for the detection of very low levels of analyte due to its extreme sensitivity and ability to speciate. In particular, IMS is widely used to detect narcotics, explosives, and chemical warfare agents, since the technique can be tailored to be particularly sensitive to compounds that form negative ions, such as nitrate-laden explosives.
As illustrated schematically in FIG. 1, IMS is based on the atmospheric pressure ionization of a sample vapor and the subsequent separation of the individual ionized components of the sample mixture via electrophoresis as they are accelerated by an external electric field gradient and transit a time-of-flight drift tube against a neutral, counter-flowing gas stream. See G. A. Eiceman and Z. Karpas, Ion Mobility Spectrometry, 2nd Ed., Chapter 4, CRC Press, Boca Raton, Fl., (2004).
The sample vapor 12 is drawn into an IMS drift tube 20 and ionized in a reaction region 22 (e.g., using a radioactive source, photoionization, or corona discharge ionizer 23), typically through proton transfer or electron capture reactions with reactant ions, to form product ions. The direction of travel of the ions depends on the polarity of the electric field 24. For example, common explosives contain electronegative nitro functional groups. Therefore, the ionization chemistry for explosives tends to form negative ions. Halogenated compounds, such as methylene chloride, can be added to a carrier gas in the reaction region 22 to provide chloride reactant ions (i.e., Cl−). The chloride reactant ions can then transfer charge to the electronegative explosive molecules to form molecular ions.
Normally, a swarm, or pulse, of ions 14 is periodically gated into the drift region 25 of the drift tube 20 by a gating means 26. In the drift region 25, the ions 14 establish a terminal velocity under the influence of the potential gradient of the electric field 24 and are separated according to their characteristic ion mobility against the counter-flowing drift gas 16. The separation begins at the entrance gate 26 and terminates at an ion detector 27 at the end of the drift region 25, where the ion response signal is recorded. For example, the ion detector 27 can be a collecting electrode or Faraday plate that records an ion response current. An aperture grid 28 can be located just ahead of the collecting electrode 27 to capacitively decouple the approaching ion cloud and prevent peak broadening due to premature response.
The response of the IMS drift tube 20 is measured as a function of ion current versus the ion arrival time at the collecting electrode 27 for a measurement cycle. The spectrum of ion arrival times at the collecting electrode 27 indicates the relative ion mobility of each ion through the drift region 25. Compound identification is based on the comparison of the mobility spectrum generated from the sample with the spectrum of a known standard.
The gating means provides a potential capture well that controls the injection of ions into the drift region. IMS drift tubes have normally been operated by opening an electrostatic ion shutter to allow a narrow pulse of ions into the time-of-flight drift region where they move toward the collecting electrode as a single ion swarm to be measured as a transient collected current. The electrostatic ion shutter can be a Bradbury-Nielson or Tyndall type shutter. The Bradbury-Nielson shutter consists of a coplanar array of parallel thin wires wherein alternated wires are connected electrically. An electrical potential is applied or removed between the neighboring wires to block or allow passage of the ion swarm through the shutter. The electric field of the Bradbury-Nielson shutter is perpendicular to the electrical field of the drift tube, thereby blocking passage of the ions into the drift region as the ions are annihilated on the coplanar wires when the electrical potential is applied to the shutter. The shutter is opened by bringing the two sets of coplanar wires to a common potential. The related Tyndall shutter uses two closely spaced planes of electrodes consisting of parallel wires or screens. A voltage is applied or removed between the planes to block or allow passage of the ion swarm.
A major deficiency of this normal operational mode is very inefficient use of available ions. Typically, the ions are annihilated during the intervals that the shutter is closed and allowed to pass as an ion swarm for only a small fraction of the time (e.g., in a 0.2 ms pulse). The ions are allowed to drift for 20-30 ms before being collected and another ion swarm is gated into the drift region. When operated in this normal mode, the duty cycle of on-to-off is generally on the order of 1% or less. Therefore, this technique requires a rather large source of ions to produce a detectable signal during the shutter open interval.
The signal-to-noise ratio (SNR) of this normal mode of operation is typically very small, but can be improved by averaging the data from many measurement cycles. As expected, for “white” noise limited signals, the SNR will increase as the square root of the number of measurements, N, according to:SNRave=√{square root over (N)}SNR1  [1]where SNR1 is the signal-to-noise ratio for a single measurement cycle and SNRave is the signal-to-noise ratio for the average measurement. The averaging approach relies on the assumption that the analyte is at a steady-state concentration in the sample vapor and is, therefore, neither varying in concentration or undergoing chemical reactions during the sample interval (i.e., during the duration of the N measurement cycles).
Another approach to IMS operation employs a Fourier transform approach (FTIMS). FTIMS uses both an entrance gate and an exit gate that are simultaneously opened and closed by a frequency sweeping square wave generator to generate a mobility interferogram. The shutter can be operated in a 50% duty cycle at the changing frequencies. The resulting ion current is then sampled and the inverse Fourier transform is performed to convert the interferogram back into the time domain. This process allows for SNR enhancement and increased resolution due to the significant increase in ion efficiency of the tube. However, this technique also requires that the chemical concentration be constant over the length of the frequency scan. See F. J. Knorr et al., Anal. Chem. 57(2), 402 (1985); R. H. St. Louis et al., Anal. Chem. 64(2), 171 (1992); and E. E. Tarver, Sensors 4, 1 (2004).
In many real-world applications of IMS, such as explosives detection, the steady-state condition cannot be relied on to be valid over long sampling intervals. For conditions where the chemical concentration is transient, application of averaging or FTIMS is limited due to finite delays that are inherent in the IMS technique. Indeed, standard averaging approaches can actually reduce the observed signal-to-noise in transient chemical systems rather than enhance it, and in extreme cases can completely mask the analyte signal. For example, an IMS system is typically gated at a rate below 50 Hz for transit times for ions of interest on the order of 20-30 msec. Thus, to achieve an improved SNR according to Eq. [1] it is assumed that the concentration is roughly constant over the sample interval (tsample)
                              t          sample                =                  N                      f            gate                                              [        2        ]            where fgate is the gating frequency. If tsample is longer than the interval in which the concentration is constant, then averaging begins to diminish the SNR rather than improve it because traces with reduced or missing signal begin to be averaged into the data. Similarly, FTIMS is limited to having the frequency scan completed before the chemical concentration changes.
Therefore, a need remains for an IMS signal-processing method having improved signal-to-noise ratio and that can be used with transient chemical signals.